ORIGINAL ARTICLE

Identifying the groundwater basin boundaries, usingenvironmental isotopes: a case study

Muhterem Demiroglu1

Received: 17 October 2013 /Accepted: 7 December 2016

� The Author(s) 2016. This article is published with open access at Springerlink.com

Abstract Groundwater, which is renewable under current

climatic conditions separately from other natural sources,

in fact is a finite resource in terms of quality and fossil

groundwater. Researchers have long emphasized the

necessity of exploiting, operating, conserving and manag-

ing groundwater in an efficient and sustainable manner

with an integrated water management approach. The

management of groundwater needs reliable information

about changes on groundwater quantity and quality.

Environmental isotopes are the most important tools to

provide this support. No matter which method we use to

calculate the groundwater budget and flow equations, we

need to determine boundary conditions or the physical

boundaries of the domain. The Groundwater divide line or

basin boundaries that separate the two adjacent basin

recharge areas from each other must be drawn correctly to

be successful in defining complex groundwater basin

boundary conditions. Environmental isotope data, as well

as other methods provide support for determining recharge

areas of the aquifers, especially for karst aquifers, resi-

dence time and interconnections between aquifer systems.

This study demonstrates the use of environmental isotope

data to interpret and correct groundwater basin boundaries

giving as an example the Yenicıkrı basin within the main

Sakarya basin.

Keywords Groundwater basin � Groundwater divide line �Recharge area � Isotope

Introduction

Groundwater management is under pressure on increased

water demand, climate change and pollution problems. The

main scientific concern is the correct understanding of the

changes in quality and quantity of groundwater caused by

human activities and climate change. Decision makers

needs modelling studies to understand and provide sus-

tainable management. The success of modelling depends

on reliable information on groundwater main parameters.

Environmental isotopes are the most important tools to

provide this support (Aggarwal et al. 2009; Surinaidu et al.

2014). Stable and unstable environmental isotopes have

been used to study hydrological systems and have been

proven particularly useful for understanding karst systems.

Among the most important areas where isotopes are useful

in groundwater applications include studies of recharge and

discharge process, flow and interconnections between

aquifers, and sources and mechanism of pollution.

Radioactive decay of environmental isotopes makes these

isotopes a unique tool for the determination of groundwater

residence time (Mazor 1991; Aggarwal et al. 2009).

Groundwater models; It is the creation of a simplified

form of the real groundwater system and the determination

of the reactions of the system to the effects by being

affected in a systematically controlled manner. These

models (analytic or numerical) may only be functional if

the process is conducted at the last part of the study and

depends upon the competence of the modeller rather than

the model itself (Voss 2011). That technique is advanta-

geous since it facilitates representing the aquifers by

dynamic models and enables authorities to assess the pro-

cess through data acquisition and updating the data, plan-

ning, adjustment, comparison and interpretation. There are

a wide range of models at present. The program

& Muhterem Demiroglu

demiroglum@itu.edu.tr

1 Geological Engineering Department, ITU Faculty of Mine,

Maslak, Turkey

123

Appl Water Sci

DOI 10.1007/s13201-016-0516-y

MODFLOW can be run by a GIS processor and has been

developed for both steady and unsteady flows and evalu-

ation of the boundary conditions. Conceptual models

yielded by those programs may be converted into appro-

priate mathematical models. One of the most important

problems during the creation of a groundwater flow model

representing a hydrogeological setting is to produce a very

real-like conceptual model. How realistic and real-like the

conceptual model is determined by usage of the accurate

and detailed input data, only then the output is realistic to

the same degree. Herein, the conceptual model term means

spatial and extensional characteristics, in other words 3-D

physical boundaries of the aquifers. Conceptualization is,

perhaps, the most critical and the most important, in

developing a groundwater model (Kresic 1997) and the

design of a conceptual model is the subject of this article.

Preparation of conceptual model requires identifying the

study area, compiling detailed information on the geolog-

ical formations, groundwater flow directions, recharge,

hydraulic parameters and sinks or sources, and deciding

appropriate boundary conditions. A basic way to evaluate

the occurrence and movement of groundwater is defining

basin or sub basin boundaries accounting for surface and

groundwater system’s inflows (recharge) and outflows

(discharge). A groundwater reservoir is defined by an

overlaying river drainage area and the underlying aquifers

that contain the stored groundwater. Surface water divide

lines identified by the highest elevations between two areas

on a map separate river basins from each other. Basin

boundaries may vary from small ridges to continental-

scale.

A groundwater divide line usually coincides with a

surface waters divide line, and that divide line also usually

matches up with a surface water draining line in granular

and unconfined aquifers (Fig. 1), while recharge areas of

the karstic and confined aquifers mostly do not coincide

with surficial drainage areas (White 2002; Gunn 2007;

Palmer 2010). Karst systems are often characterized by

substantial groundwater–surface water connections via

processes such as aquifer recharge by losing streams,

fracture and conduit connections between surface water

and groundwater, and spring flow contributions to surface

water (e.g., Katz et al. 1997; Musgrove et al. 2010).

As can be seen in Fig. 2, it is not easy for defining the

basins which have complex hydrogeological properties.

Specifying the groundwater divide line accurately and

defining a grid system to which the boundary conditions

and numerical simulations are based, and forms the basis of

a modelling practice (Unlu et al. 2004). Yenicıkrı basin,located in the upper Sakarya basin in central Turkey is one

of the examples representing that case (Fig. 2). High-

yielding wells opened by the State Hydraulic Works in the

small river basin, Yenicıkri River, and Yenicıkrı spring

meet the drinking and irrigation water needs of the

Kayakent township. Prepared water budget calculations

using basin boundaries did not match with this rich aquifer.

Shortly after the wells in the marbles were put into oper-

ation, Yenicıkrı spring (47 l/s) dried up. Environmental

isotope techniques were performed for determining the

recharge area and residence time of groundwater for cre-

ating conceptual model.

Study area and hydrogeology

The units outcropping at Yenicıkrı basin, from bottom to top,

respectively, Paleozoic marble, which is the main reservoir

rock for hot and cold water, are bordered by impermeable

schists. Due to continually effective uprising and erosional

processes, the marble outcroppings at the area are not so

thick. Yet, depending on the low-dissolvability and being

enclosed by impermeable zones such as granites and schists

at the bottom, the marbles display no remarkable karstic

formations at present. Characterized by the presence of well-

developed karstic-reservoirs in Yenicıkrı area, it has beeninterpreted that these structures were included in hydrolog-

ical cycle as a result of step faults. Thereafter, the evaporitic-

lacustrian and tectonically active period terminated and all

the units in the region, both the Miocene-aged and the older

ones, have undergone erosional processes. Karstification in

the carbonates began to develop in that time span.

Emplacement of the Sakarya River is the primary factor

controlling the energy gradient in the area (Ekmekci 2003).

Marble, at the top of the metamorphic series, at higher

elevations of the region, represents the upper part of the

aquifer system and the lower part of the aquifer system as a

result of step faults. Yenicıkrı spring recharges, circulates

and discharges from marbles and wells cut only in marbles.

The marbles are overlain by the young units uncon-

formably. They contain terrestrial detritics, light-colored

and include laterally and vertically gradational silts, clays,

gypsum, marls and mudstones (Fig. 3). The Neogene units

are classified as very poor aquifer. According to the flow

measurements made between 1986 and 2005, the average

flow rate of Yenicıkrı spring is 47 lt/s. The static level of

wells is the same as the spring discharge elevation (890

meters). It can be said that Yenicıkrı spring works like a

spillover outlets. As a result of the operation of the wells,

the water level has fallen and the spring has dried up. Pump

tests in the marbles yielded hydraulic conductivities from

1.19 to 98.9 m/day and specific capacity ranges from 0.64

to 75 l/s/m (Demiroglu et al. 2011).

As seen in Fig. 2, the drainage divide line at the Yeni-

cıkrı Basin passes through marbles displaying karstic

characteristics, and for the determination of the recharge

elevation environmental isotope techniques were

Appl Water Sci

123

performed (Demiroglu 2008). Hydrogeological cross sec-

tions indicate the extent and continuity of lithologies

(Fig. 3) and the discontinuities which can link the aquifer

systems. Yenicıkrı basin boundaries depend upon the

extent of the lithologies were re-delineated and combined

with Gunyuzu basin after detailed hydrogeology, tectonics

and isotope studies.

Materials and methods

The technique applied generally during these studies is to

create a Digital Elevation Model (DEM) of the studied area

and make an output model subject to the ArcMap

Hydrologic Modelling Extension (ESRI) application, and

so to yield a map outlining basin water divide lines

(Fig. 4). This boundaries didn’t match with lithological

groundwater boundaries and it is required to make some

changes on borders using hydrogeological and hydro-

chemical data. Isotope techniques were applied in the

determination of recharge areas more accurately.

Among the environmental isotopes participating in the

hydrological cycle at different stages and included in the

waters naturally, they occur naturally in water bodies and

do not change during movement of groundwater. So these

are used as tracers in both hydrology and hydrogeology

(Tezcan 2002; Aggarwal et al. 2009). These isotopes can

be measured by a specific type of mass spectrometer with

Fig. 1 An unconfined aquifer

boundary conditions (Fetter

2001)

Fig. 2 Schematic illustration

the hydrogeological conceptual

model of the Yenicıkrı Basin

Appl Water Sci

123

required sensibility equipment. Measured d18O and d2Hfigures do not reflect absolute concentrations, but just point

out deviation (as per thousand) of isotopic rates from

Standard Mean Ocean Water (SMOW).

Isotopic rates are expressed as d notation as below:

d &ð Þ¼ Rsample �Rstandard

� �=Rstandard � 1000

Fig. 3 Spatial cross section view of the study area

Fig. 4 a Drainage areas of Yenicıkrı and surrounding basins (drawn by classical method). b Drainage areas representation drawn on Digital

Elevation Model (DEM)

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123

The d is a relative value. In other words, the d value

reflects discrepancies of 18O/16O or D/H ratios of the

sample from 18O/16O or D/H ratios of a standard. Through

water analyses, accepted standard for isotopes is SMOW

(Standard Mean Ocean Water) (Cifter and Sayın 2004;

Aggarwal et al. 2009).

From previous works on isotope hydrology, it has been

concluded that various factors such as geographic location,

latitude, temperature, etc. affect heavy isotopes concen-

tration of water vapor. It is a finding that, the d18O content

in atmospheric water vapor decreases toward continental

and higher areas. Since temperature is one of the basic

factors controlling evaporization, the concentration of

heavy isotopes increases at higher temperature periods.

Similarly, precipitations in the summer season have heavier

isotopic composition than those of the winter season.

Heavy isotope concentration of heavy rains is lower than

those of showers. Heavy isotope content decreases gradu-

ally toward terrestrial areas in comparison with oceans and

seas. Besides these parameters, heavy isotope concentra-

tion of the waters falls principally as the elevation

increases; in other words, heavy isotopes concentration in

the waters is in relation to altitude with an inversely pro-

portional manner. Due to local climatic conditions, frontal

precipitation system and the topography controlling the

precipitation, the d18O content of atmospheric water

decrease ranges from -0.15 to -0.5% per 100 m rise in

altitude (Clark and Fritz 1997).

Springs discharging at different elevations and display-

ing seasonal variations in their flow rates (i.e., drying in

summer period and yield increase in flow rates in rainy

seasons) can also be made functional in establishing the

relation between d18O or d2H and altitude when there has

been no data representing precipitation and being gathered

from different elevations. Gocenoluk, Akpınar, Gumusbel

and Musluk pınarı sample springs from different altitudes

in nearby basins were used to represent precipitation for the

Yenicıkrı basin. Several factors may complicate the use of

a mean annual d18O value as an indicator of recharge

elevations and may overestimate recharge elevations (Ab-

bott et al. 2000). Groundwater flowing from the highest

altitude of the spring’s recharge area (lowest mean d 18O

values) may become mixed with water recharged at lower

elevations (higher mean d18O values) along the flow path,

as well as mixing with old water coming from deep cir-

culation. Moreover, d18O content of hot waters increases to

some extent as a result of transfer between water and rock

in higher temperature conditions. This interaction is evi-

dent at very high temperatures, 150 �C (Truesdell and

Hulston 1980; Yildirim and Guner 2002). Using d2H-alti-tude relations are preferred to specify the altitude of

recharge areas for geothermal springs since there is no

variation in d2H concentration of water in higher

temperature water–rock interaction processes. Clark and

Fritz (1997) has reported that the changing rate in d2Hisotope concentration varies between -1 to -4% per

100-m rise in altitude.

Tritium (3H), an unstable, a short-lived isotope of

hydrogen with a half-life of 12.3 years, is a commonly

used tracer for determining the age of groundwater less

than fifty years old. Tritium concentrations are expressed in

tritium units (TU), where one TU indicates that one of the

1018 hydrogen atoms is the Tritium (3H) atom (Clark and

Fritz 1997; Fetter 2001). Tritium is produced naturally in

the stratosphere by the effects of cosmic radiation on 14N,

and enters the hydrologic cycle via precipitation. Large

volumes of anthropogenic tritium were produced between

1951 and 1962 by atmospheric testing of thermonuclear

weapons. Use of the tritium isotope for dating modern

groundwater now relies upon the largely natural 3H signal

(Land and Huff 2009). Tritium concentrations in precipi-

tation sample or seasonal, low circulated, small springs

discharging at different elevations, can accept modern or

current groundwater. A mean 12 TU value in Yenicıkrıbasin are considered to be modern groundwater. Carbon-14

(14C) is a commonly used tracer for determining the age of

groundwater more than fifty years old. Carbon isotope is a

naturally occurring radioactive isotope of carbon, and was

also produced by atmospheric thermonuclear testing during

the mid-twentieth century, similar to Tritium. The 14C

radioactivity of inorganic carbon in water is expressed in

terms of Percent Modern Carbon (PMC) relative to the

abundance of 14C in atmospheric CO2. Once isolated from

atmospheric gases, the abundance of 14C decreases at a rate

governed by its half-life of 5730 years (Land and Huff

2009).

Results and discussions

Gocenoluk, Akpınar, Gumusbel and Musluk pınarı springsfrom different elevations in nearby basins were used to

represent precipitation for the Yenicıkrı basin and the d18Ochanging rate for increasing elevation has been determined

as -0.32% per 100 m rise in altitude (Demiroglu 2008;

Demiroglu et al. 2011).

Accordingly, the decrease in d18O concentration for

each 100 m increase in altitude is -0.32%. Data on the

d18O content in Yenicıkrı spring gave rise to a conclusion

that the altitude of the catchment area of that spring is

about 1465 m (Fig. 4). It shows that the recharge areas of

the Yenicıkrı spring extend into Arayit Mountain, distant

from the water divide line of the Yenicıkrı basin that this

spring lies within (Fig. 5).

The mean d2H values on the contrary point out an ele-

vation reaching to 2276 m (Fig. 6). In this region the

Appl Water Sci

123

present topography pictures indicates ridges reach up to

only 1817 m. In that respect data gained from isotopes

points out that, the catchment area of the Yenicıkrı basinextends into Arayit Mountain (1.817) and the presence of

static water stored under quite different conditions than

those actually prevailing. Tritium concentrations in the

groundwater range from 12 to 2.3 TU, indicating that

groundwater residence times may vary from less than

1 year to in excess of 30 years. But in reality the aquifer is

a mixture of two different deep and shallow circulating

waters. The age of the deep-circulated ground waters was

determined to be 10,000 years by 14C analysis in the

Cardak hamamı spring which occurs in the same karstic

system (Demiroglu and Orgun 2014).

Heavy isotopes were also used to specify the elevation

of the catchment area of Sakarbası (Sivrihisar/Eskisehir)springs, discharging at lower altitudes and located within

the boundaries of main Sakarya drainage area (Gunay

2006). Reflecting a discharge of a 300 m height, Kırkgo-zeler (Antalya) spring group has a catchment area reaching

to heights varying from 1918 to 2827 m (Atilla 1996).

Gunay and Bayarı (1989) has concluded that the decrease

reaches up to -0.25% for each 100 m-height in Koprucay

Basin. Likewise, the decrease in the 18O values between

-0.15 and -0.25% for the same change in elevation has

been determined at the Gokova karstic springs area by

Kurttas (1997). These results make environmental isotopes

unique tools for the determination of groundwater basin

boundaries.

Conclusions

The measurement of environmental isotopes in addition to

evaluate the lithological and structural features gave rise to

a conclusion that catchment areas of the springs extend into

regions well distant from water divide lines of the basins

that these springs lie within.

In that context, one can say that, for specifying the

catchment areas of the basins displaying complex boundary

conditions due to complex geological structure as in the

case of the Yenicıkrı Basin, it is required to make func-

tional the stable isotopes techniques.

Acknowledgements The manuscript has been edited in terms of its

language by Darwin E. Fox, whom the authors deeply thank for the

helpful comments improving the manuscript.

Open Access This article is distributed under the terms of the

Creative Commons Attribution 4.0 International License (http://

creativecommons.org/licenses/by/4.0/), which permits unrestricted

use, distribution, and reproduction in any medium, provided you give

appropriate credit to the original author(s) and the source, provide a

link to the Creative Commons license, and indicate if changes were

made.

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